On the boil: how Toyota is paving a new hydrogen path

Hydrogen development today tends to focus on more prosaic transport, such as delivery vehicles, but Toyota’s projects err to the exciting.

Back in the 2021 Fuji 24 Hours race, it fielded a Corolla Sport powered not by a hydrogen fuel cell but by a specially developed H2 version of the three-cylinder GR Yaris combustion engine, and last month the car competed in the final of the Super Taikyu Series.

The Corolla runs on ‘boil-off’ hydrogen from liquid hydrogen carried in its fuel tank.

Burning hydrogen in a combustion engine generates no emissions of carbon monoxide (CO) or unburned hydrocarbons (HC), which are two of the three emissions cleaned up in a conventional petrol car by a three-way catalytic converter.

The third comprises oxides of nitrogen (NOx), and although the racing engine still produces some of that, the quantities are far lower than those produced by a petrol ICE, and they are neutralised by exhaust after-treatment.

The novel approach to using boil-off gas stems from liquid hydrogen’s unavoidable Achilles heel, which is that it literally evaporates while the car is standing doing nothing. Liquid hydrogen is stored in cryogenic tanks at a chilly -253deg C. Inevitably heat from the outside penetrates the insulated tank and the hydrogen begins to evaporate – it boils off.

A couple of decades ago, when modern hydrogen fuel cell and combustion engines began serious development, the high-pressure tanks for storing gaseous hydrogen at 350-700 bar, the go-to technology today, hadn’t emerged.

Most projects that focused on using liquid hydrogen and dealing with boil-off meant releasing it through a valve as pressure increased, which wasted a percentage of the fuel on a continual basis. If a car was left standing for long enough, it could run out of fuel without moving.

On the plus side, liquid hydrogen is far more energy-dense than gaseous hydrogen, with the potential for greater range.

What Toyota has done is turn boil-off into a positive: instead of simply venting the evaporating gas to the atmosphere, it captures it and uses it in several stages.

Boil-off gas is pressurised and fed to the engine and used directly as fuel to power the car. The pump that does the work increases the pressure of the boil-off gas by between two and four times, which is enough to fuel the engine.

Left-over gas is used to generate electricity in a hydrogen fuel cell stack, which is used to power a liquid hydrogen pump and other ancillaries. If there’s still a surplus after those steps, the excess is converted to water vapour using a catalyst and safely released outside the car.

Toyota is looking to form technical partnerships to develop the technology further.

This really cool idea (pun intended) emphasises the staggering levels of ingenuity that continue to emerge from the engineering departments of car makers worldwide.